1. Field of the Invention
The present invention mainly relates to a magneto-resistive element, and particularly relates to the structure of a cap layer of a CPP-GMR element.
2. Description of the Related Art
In accordance with the development of hard disk drives (HDD) toward higher density, a thin-film magnetic head having improved performance has been required. A composite thin-film magnetic head, in which a read head portion having a magneto-resistive element (MR element) for reading data and a write head potion having an inductive electromagnetic transducer for recording data are stacked, is widely used for a thin-film magnetic head. A magneto-resistive element is arranged inside insulating films (inter-shield insulating films) which are provided between first and second shield layers. The inter-shield insulating films are provided to electrically insulate the magneto-resistive element from the first and second shield layers in order to prevent leakage of sense current.
With the increase in recording density, the need for a narrower shield gap and a narrower track width has been increasing. To cope with this requirement, Japanese Patent Application Laid-Open Publication No. 288807/97 discloses a head structure in which first and second shield layers and a magneto-resistive element are electrically connected in series to make the inter-shield insulation films unnecessary. In this structure, since the magneto-resistive element is in contact with the shield layers via non-magnetic metal layers, a narrower shield gap is achieved with lesser possibility for dielectric breakdown between shield layers. Such a magneto-resistive element is called a CPP-GMR (Current Perpendicular to the Plane Giant Magneto-Resistance) element because it operates with electric current that flows in a direction perpendicular to layer surfaces. A CPP-GMR element is promising as a magneto-resistive element for a read head portion that requires a track width of 0.1 μm or less and a height of 0.1 μm or less.
The structure in which a magneto-resistive element is connected to shield layers via metal layers has the advantage that it has improved heat radiation efficiency and a capability for higher operating current. Further, if the element has a small cross section, then it exhibits a larger electric resistance and, at the same time, a larger change in resistance. In other words, a CPP-GMR element has the advantage that it is suitable for achieving a narrow track width.
However, it is reported that when a large amount of electric current is applied to operate a CPP-GMR element, magnetic noise due to “spin injection effect” occurs. For example, refer to “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current” by L. Berger, Physical Review B (USA), American Physical Society, page 54, No. 13, vol. 54, October, 1996, and “Current-driven Excitation of Magnetic Multilayers” by J. C. Slonczewski, Journal of Magnetism and Magnetic Materials (USA), Elsevier Science B. V., pages L1 to L7, vol. 159, 1996.
The spin injection effect is a phenomenon that spin-polarized conduction electrons in the pinned layer enter the free layer via a non-magnetic spacer layer (e.g., a Cu layer), and interact with electrons that govern the magnetic state of the free layer, thereby disturbing the magnetic state of the free layer.
FIGS. 1, 2A to 2C are schematic diagrams for illustrating the spin injection effect. FIG. 1 is a schematic diagram showing an essential part of the film structure of a CPP-GMR element, and FIGS. 2A to 2C are schematic diagrams showing magnetization states of a free layer. In a CPP-GMR element, pinned layer 104, spacer layer 105 consisting of a non-magnetic and conductive layer, and free layer 106 are formed adjacent to each other in this order, as shown in FIG. 1. The magnetization of pinned layer 104 is fixed in magnetization direction S1 irrespective of the external magnetic field. Free layer 106 is magnetized in magnetization direction S2 under the influence of a bias magnetic field which is generated by hard magnetic layers, not shown, that are arranged on both sides of free layer 106. The magnetization direction of free layer 106 is rotated by external magnetic field that is generated by a recording medium, not shown. Magneto-resistance is changed in accordance with the amount of rotation, and thereby magnetic information is read from the recording medium.
Cap layer 107, which is made of, for example, tantalum (Ta) or titanium (Ti), is arranged in contact with free layer 106. Cap layer 107 is provided to protect the surface of a wafer during manufacturing processes, and to ensure workability when a bar is lapped to form an air bearing surface, which is the surface of a thin-film magnetic head that faces a recording medium. Cap layer 107 may also be made of hafnium (Hf), niobium (Nb), zirconium (Zr), molybdenum (Mo), tungsten (W) etc. See, for example, Japanese Patent Application Laid-Open Publication No. 2003-218428.
First electrode layer 111 is formed in contact with cap layer 107. Second electrode layer 112 is formed on the side of pinned layer 104. The first and second electrode layers, which are formed of, for example, NiFe, have the function of providing magnetic shields for free layer 106, as well as the function of electrodes for applying electric current to pinned layer 104, spacer layer 105, and free layer 106 in a direction perpendicular to the layer surfaces.
When sense current C is applied, conduction electrons flow in direction D that is opposite to the direction of the current. Conduction electrons having spins in various directions are supplied from second electrode 112. When the conduction electrons reach pinned layer 104, conduction electrons having a spin direction that is directed to the same direction as magnetization direction S1 of pinned layer 104 (called conduction electrons e1 hereinafter) are apt to pass through pinned layer 104 by virtue of the spin polarization effect of pinned layer 104. However, conduction electrons having a spin direction that is directed to the opposite direction are less apt to pass through pinned layer 104, because they are reflected inside pinned layer 104. As a result, conduction electrons e1 having a spin direction that is directed to the same direction as magnetization direction S1 are allowed to pass through pinned layer 104 (spin polarized), and to enter free layer 106 via spacer layer 105.
As described above, free layer 106 is magnetized in magnetization direction S2, which is parallel to the layer surface and is perpendicular to magnetization direction S1 of pinned layer 104. Assume that free layer 106 is magnetized in magnetization direction S3 that is inclined by some degrees to magnetization direction S2, as shown in FIG. 2A. When conduction electrons e1 are injected, the magnetization direction of each magnetic domain, like precession of a spinning top, begins to rotate due to interaction with electrons that govern the magnetic state of free layer 106, and, at the same time, each magnetic domain is subject to torque F which gradually directs the magnetization direction of the magnetic domain toward magnetization direction S1. Since torque F is directed to magnetization direction S1 at any time, regardless of the magnetization direction of the magnetic domain, the magnetization direction of the magnetic domain is finally aligned with magnetization direction S1, as shown in FIGS. 2B and 2C. Torque F is proportional to the quantity of conduction electrons e1 that have been injected, i.e., the magnitude of sense current. The above phenomenon is called magnetization reversal by spin injection, or spin transfer, and torque F is called spin transfer torque.
This phenomenon implies that the magnetization state of the free layer may vary without being affected by an external magnetic field. This phenomenon may generate noise, and is disadvantageous for developing a more highly sensitive magnetic head.
A dual spin valve structure is proposed as a means for canceling the spin injection effect. For example, see “Current Induced Noise in CPP Spin valves” by J. Z. Zhu et al., IEEE Transactions on Magnetics, USA, pp. 2323-2325, No. 4, vol. 40, July, 2004. This structure enables spin-polarized conduction electrons that flow from a pinned layer on one side to efficiently remove the influence of conduction electrons that flow from another pinned layer on the other side.
However, a dual spin valve, in which a free layer is sandwiched by two pinned layers, tends to have a larger film thickness due to the additional pinned layer. Since the direction of the film thickness of a spin valve used in a thin-film magnetic head is aligned to the circumferential direction of a recording medium, it is difficult for a film structure having a large film thickness to achieve a short recording pitch in the circumferential direction of a recording medium, or a narrow shield gap. However, a film structure, other than a dual spin valve, that can reduce noise caused by spin injection effect has not yet been proposed.